We will test the relationship between the presence of NMDA receptors in the visual cortex and the segregation of afferents from the lateral geniculate to the cortex. We have shown that the contribution of NMDA receptors to the visual response in layers IV, V and VI of cat visual cortex drops between 3 and 6 weeks of age, slightly preceding the segregation of geniculocortical afferents. This finding, coupled with the implication of NMDA receptors in afferent segregation in the three-eyed frog (Cline et al., 1987) leads to the hypothesis that NMDA receptors may play a similar role in the visual cortex. We have devised a series of experiments to test this hypothesis, and to test the suggestion that the elimination of NMDA receptors from the postsynaptic terminal may precede the withdrawal of the afferent terminal, just as there is a loss of acetylcholine receptors from the postsynaptic site at the neuromuscular junction preceding the withdrawal of the nerve overlying the junction (Rich and Lichtman, 1989).